The present invention relates to an electron-beam generating apparatus having a multi-electron-beam source in which a plurality of cold cathode devices are wired in a matrix, an image display apparatus using the electron-beam generating apparatus, and a method of driving these apparatuses.
Conventionally, two types of devices, namely thermionic and cold cathode devices, are known as electron-emitting devices. Examples of cold cathode devices are surface-conduction electron-emitting devices, field-emission-type devices (to be referred to as FE-type devices hereinafter), and metal/insulator/metal type emission devices (to be referred to as MIM-type devices hereinafter).
A known example of the surface-conduction electron-emitting devices is described in, e.g., M. I. Elinson, et al., "The Emission of Hot Electrons and the Field Emission of Electrons from Tin Oxide," Radio. Eng. Electronic Phys., 10, 1290 (1965) and other examples to be described later.
The surface-conduction electron-emitting device utilizes the phenomenon in which electron emission is caused in a small-area thin film formed on a substrate, by providing a current parallel to the film surface. The surface-conduction electron-emitting device includes devices using an Au thin film (G. Dittmer, "Electrical Conduction and Electron Emission of Discontinuous Thin Films," Thin Solid Films, 9,317 (1972)), an In.sub.2 O.sub.3 /SnO.sub.2 thin film (M. Hartwell and C. G. Fonstad, "Strong Electron Emission From Patterned Tin-Indium Oxide Thin Films," IEEE Trans. ED Conf., 519 (1975)), and a carbon thin film (Hisashi Araki, et al., "Electroforming and Electron Emission of Carbon Thin Films," Vacuum, Vol. 26, No. 1, p. 22 (1983)), and the like, in addition to an SnO.sub.2 thin film according to Elinson mentioned above.
FIG. 23 is a plan view of the surface-conduction emitting device according to M. Hartwell et al. as a typical example of the structures of these surface-conduction electron-emitting devices. Referring to FIG. 23, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped plane pattern, as shown in FIG. 23. An electron-emitting portion 3005 is formed by performing an electrification process (referred to as an energization forming process to be described later) with respect to the conductive thin film 3004. Referring to FIG. 23, a spacing L is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. The electron-emitting portion 3005 is shown in a rectangular shape at the center of the conductive thin film 3004 for the sake of illustrative convenience, however, this does not exactly show the actual position and shape of the electron-emitting portion.
In the above surface-conduction electron-emitting device by M. Hartwell et al., typically the electron-emitting portion 3005 is formed by performing the electrification process called energization forming process for the conductive thin film 3004 before electron emission. According to the energization forming process, electrification is performed by applying a constant or varying DC voltage which increases at a very slow rate of, e.g., 1 V/min, to both ends of the conductive thin film 3004, so as to partially destroy or deform the conductive thin film 3004 or change the properties of the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 or part where the properties are changed has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the energization forming process, electron emission occurs near the fissure.
Known examples of the FE-type devices are described in W. P. Dyke and W. W. Dolan, "Field Emission", Advances in Electronics Electron Physics, 8,89 (1956) and C. A. Spindt et al., "Physical Properties of Thin Field Emission Cathodes with Molybdenum Cones", J. Appl. Phys., 47,5248 (1976).
FIG. 24 is a cross-sectional view of the device according to C. A. Spindt et al. as a typical example of the construction of the FE-type devices. Referring to FIG. 24, reference numeral 3010 denotes a substrate; 3011, an emitter wiring comprising an electrically conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. The device is caused to produce field emission from the tip of the emitter cone 3012 by applying an appropriate voltage across the emitter cone 3012 and gate electrode 3014.
In another example of the construction of an FE-type device, the stacked structure of the kind shown in FIG. 24 is not used. Rather, the emitter and gate electrode are arranged on the substrate in a state substantially parallel to the plane of the substrate.
A known example of the MIM-type is described by C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961). FIG. 25 is a sectional view illustrating a typical example of the construction of the MIM-type device. Referring to FIG. 25, reference numeral 3020 denotes a substrate; 3021, a lower electrode consisting of metal; 3022, a thin insulating layer having a thickness on the order of 100 .ANG.; and 3023, an upper electrode consisting of metal and having a thickness on the order of 80 to 300 .ANG.. The device is caused to produce field emission from the surface of the upper electrode 3023 by applying an appropriate voltage across the upper electrode 3023 and lower electrode 3021.
Since the above-mentioned cold cathode device makes it possible to obtain electron emission at a lower temperature in comparison with a thermionic cathode device, a heater for applying heat is unnecessary. Accordingly, the structure is simpler than that of the thermionic cathode device and it is possible to fabricate devices that are finer. Further, even though a large number of devices are arranged on a substrate at a high density, problems such as fusing of the substrate do not easily occur. In addition, the cold cathode device differs from the thermionic cathode device in that the latter has a slow response because it is operated by heat produced by a heater. Thus, an advantage of the cold cathode device is the quicker response.
For these reasons, extensive research into applications for cold cathode devices is being carried out.
By way of example, among the various cold cathode devices, the surface-conduction electron-emitting device is particularly simple in structure and easy to manufacture and therefore is advantageous in that a large number of devices can be formed over a large area. Accordingly, research has been directed to a method of arraying and driving a large number of the devices, as disclosed in Japanese Patent Application Laid-Open No. 64-31332, filed by the present applicant.
Further, applications of surface-conduction electron-emitting devices that have been researched are image forming apparatuses such as an image display apparatus and an image recording apparatus, charged beam sources, and the like.
As for applications to an image display apparatus, research has been conducted with regard to such an image display apparatus using, in combination, surface-conduction electron-emitting devices and phosphors which emit light in response to irradiation by an electron beam, as disclosed, for example, in the specifications of U.S. Pat. No. 5,066,883 and Japanese Patent Application Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant. The image display apparatus using the combination of the surface-conduction electron-emitting devices and phosphors is expected to have characteristics superior to those of the conventional image display apparatus of other types. For example, in comparison with liquid-crystal display apparatuses that have become so popular in recent years, the above-mentioned image display apparatus is superior since it emits its own light and therefore does not require back-lighting. It also has a wider viewing angle.
A method of driving a number of FE-type devices in a row is disclosed, for example, in the specification of U.S. Pat. No. 4,904,895 filed by the present applicant. A flat-type display apparatus reported by R. Meyer et al., for example, is known as an example of an application of an FE-type device to an image display apparatus. [R. Meyer: "Recent Development on Microtips Display at LETI", Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6.about.9, (1991).]
An example in which a number of MIM-type devices are arrayed in a row and applied to an image display apparatus is disclosed in the specification of Japanese Patent Application Laid-Open No. 3-55738 filed by the present applicant.
The present inventors have examined electron-emitting devices according to various materials, manufacturing methods, and structures, in addition to the above conventional devices. The present inventors have also studied a multi-electron-beam source in which a large number of electron-emitting devices are arranged, and an image display apparatus to which this multi-electron-beam source is applied.
The present inventors have also examined a multi-electron-beam source according to an electric wiring method shown in FIG. 26. More specifically, this multi-electron-beam source is constituted by two-dimensionally arranging a large number of electron-emitting devices and wiring these devices in a matrix, as shown in FIG. 26.
Referring to FIG. 26, reference numeral 4001 denotes an electron-emitting device; 4002, a row wiring; and 4003, a column wiring. In reality, the row wiring 4002 and the column wiring 4003 include limited electrical resistance; yet, in FIG. 26, they are represented as wiring resistances 4004 and 4005. The wiring shown in FIG. 26 is referred to as simple matrix wiring.
For illustrative convenience, the multi-electron-beam source constituted by a 6.times.6 matrix is shown in FIG. 26. However, the scale of the matrix is not limited to this arrangement. In a multi-electron-beam source for an image display apparatus, a number of devices sufficient to perform the desired image display are arranged and wired.
In the multi-electron-beam source in which the electron-emitting devices are wired in a simple matrix, appropriate electrical signals are supplied to the row wiring 4002 and the column wiring 4003 to output desired electron beams. For instance, when the electron-emitting devices of one arbitrary row in the matrix are to be driven, a selection voltage V.sub.s is applied to the row wiring 4002 of the selected row. Simultaneously, a non-selection voltage V.sub.ns is applied to the row wiring 4002 of unselected rows. In synchronization with this operation, a driving voltage V.sub.e for outputting electron beams is applied to the column wiring 4003. According to this method, a voltage (V.sub.e -V.sub.s) is applied to the electron-emitting devices of the selected row, and a voltage (V.sub.e -V.sub.ns) is applied to the electron-emitting devices of the unselected rows, assuming that a voltage drop caused by the wiring resistances 4004 and 4005 is negligible. When the voltages V.sub.e, V.sub.s, and V.sub.ns are set to appropriate levels, electron beams with a desired intensity are output from only the electron-emitting devices of the selected row. When different levels of driving voltages V.sub.e are applied to the respective column wiring 4003, electron beams with different intensities are output from the respective devices of the selected row. Since the response rate of the cold cathode device is fast, the period of time over which electron beams are output can also be changed in accordance with the period of time for applying the driving voltage V.sub.e.
Accordingly, the multi-electron-beam source having electron-emitting devices arranged in a simple matrix can be used in a variety of applications. For example, the multi-electron-beam source can be suitably used as an electron source for an image display apparatus by appropriately supplying a voltage signal according to image data.
However, when a voltage source is actually connected to the multi-electron-beam source and the multi-electron-beam source is driven in the above described method of voltage application, a problem arises in that the voltage practically supplied to each of the electron-emitting devices is varied since the voltage drops due to wiring resistance.
A primary cause of such variance in the voltage applied to each of the devices is the difference in wiring lengths for each of the electron-emitting devices wired in a simple matrix (i.e., magnitudes of wiring resistances are different for each of the devices).
The second cause is the non-uniform voltage drop caused by the wiring resistance 4004 in respective portions of the row wiring. Since the current flowing from the row wiring of the selected row is diverged to each of the electron-emitting devices connected to the selected row, levels of the current provided to each of the wiring resistances 4004 are not uniform, causing the aforementioned non-uniformity.
The third cause is in that the level of voltage drop caused by the wiring resistance varies depending on a driving pattern (an image pattern to be displayed). This is because the current provided to the wiring resistance changes in accordance with a driving pattern.
Due to the aforementioned causes, the voltage applied to each of the electron-emitting devices varies. Therefore, an intensity of an electron beam outputted from each of the electron-emitting devices deviates from a desired value, causing a problem in applications. For instance, in a case where the above-described method is applied to an image display apparatus, luminance of a displayed image becomes non-uniform, or the luminance changes depending on a displayed image pattern.
Furthermore, since the variance of voltage tends to be greater as the scale of the simple matrix becomes large, the number of pixels in the image display apparatus has to be limited.
In view of the above problems, the present inventors have conducted extensive studies and have experimented with a driving method different from the aforementioned voltage application method.
More specifically, according to the experimental method, upon driving a multi-electron-beam source in which the electron-emitting devices are wired in a simple matrix, instead of connecting the voltage source with the column wiring to apply the driving voltage V.sub.e, a current source is connected to supply a current necessary to output desired electron beams. In this method, the level of emission current I.sub.e is controlled by controlling the level of device current I.sub.f.
In other words, the level of device current I.sub.f to be provided to each electron-emitting device is determined by referring to a characteristic representing (device current I.sub.f) vs. (emission current I.sub.e) of the electron-emitting device, and the determined level of the device current I.sub.f is supplied by the current source connected to the row wiring. More specifically, the driving circuit is constructed by combining electric circuits such as a memory storing the characteristic representing (device current I.sub.f) vs. (emission current I.sub.e), a calculator for determining the device current I.sub.f to be provided, a controlled current source and the like. The controlled current source of the driving circuit may employ a form of a circuit in which the level of the device current I.sub.f to be provided is first converted to a voltage signal and then to current by a voltage/current converter.
According to the above current source method, as compared with the foregoing driving method of connecting a voltage source, it is less likely to be influenced by voltage drop due to the wiring resistance. Therefore, the above method provides a considerable effect to minimize the variance and change in intensity of output electron beams (EPA 688 035).
However, the driving method of connecting a current source still raises the following problems.
That is, in a case where a constant current pulse having a short time-width is supplied from a controlled constant current source to the multi-electron-beam source in which a considerably large number of electron-emitting devices are wired in a matrix, an electron-beam is hardly emitted. If the constant current pulse is continuously supplied for a relatively long period of time, electron-beams are emitted as a matter of course; however, a long start-up time is necessary to start the electron emission.
FIGS. 22B-22E are time charts for explaining the above. FIG. 22B is a graph showing timing for scanning the row wiring; FIG. 22C, a graph showing a current waveform output from the controlled constant current source; FIG. 22D, a graph showing the driving current practically provided to the electron-emitting devices; and FIG. 22E, a graph showing the intensity of electron beams emitted from the electron-emitting devices. As can be seen from these figures, when a short current pulse is supplied from the controlled constant current source, device current I.sub.f is not provided to the electron-emitting devices. If a long current pulse is supplied, the driving current provided to the electron-emitting devices has a waveform with a large rise-time.
Although a cold cathode type electron-emitting device has a characteristic of fast response, since the current waveform has a long rise time, the resulting waveform of the emission current I.sub.e is also deformed.
The foregoing problems arise due to the following reasons. In a multi-electron-beam source where electron-emitting devices are wired in a simple matrix, parasitic capacity increases as the scale of the matrix is enlarged. The parasitic capacity is mainly present where the row wiring and column wiring intersect. An equivalent circuit thereof is shown in FIG. 22A. When a controlled constant current source 11 connected to a column wiring 54 starts supplying a constant current I.sub.1, the supplied current is first consumed to charge parasitic capacity 48 before the supplied current serves as a driving current for electron-emitting devices 41. Thus, the practical response speed of the electron-emitting devices is reduced.
More specifically, to attain practical light emission luminance in a display apparatus having cold cathode devices and phosphors, it is necessary to supply, generally speaking, at least 1 .mu.A to 10 mA of driving current, to a cold cathode device corresponding to one pixel. If a driving current larger than necessary is supplied, a problem arises in that the life of the cold cathode devices is shortened.
To cope with the above problems, an output current of the controlled constant current source is controlled to an appropriate value ranging from 1 .mu.A to 1 mA. (In reality, the most appropriate value of driving current is determined in consideration of the type, material, and the form of the cold cathode, or efficiency of light emission and an acceleration voltage of the phosphors.)
Meanwhile, in order to serve as a practical television set or a computer display, it is preferable to have, e.g., the number of pixels of a display screen equal to more than 500.times.500 and a screen whose diagonal size is larger than 15 inches. If the matrix wiring is to be formed by utilizing a general technique of deposition, wiring resistance r and parasitic capacity c are produced, as has been described above. The circuit has a charging time constant Tc which depends upon the magnitude of r and c. (Strictly speaking, the time constant of the circuit also depends upon plural parameters, as a matter of course.)
In the case of driving the electron-emitting devices with the voltage source, the response speed of the electron-emitting devices which are connected in parallel to the parasitic capacity depends upon the time constant Tc.
However, in a case where a constant current ranging from 1 .mu.A to 1 mA is supplied by the controlled current source as described above, the time necessary for charging is even longer than the above time constant Tc. In other words, the practical response speed of the electron-emitting devices is slower than that in the case of driving by a voltage source.
Accordingly, in a case where light emission luminance in a display apparatus is controlled by the pulse-width modulating method, linearity of a grayscale in a low luminance portion is deteriorated. Moreover, when an image moving in quick motion is displayed, a viewer receives an unnatural image.
As described above, in the case where a modulated signal is supplied by a controlled constant current source, the influence of voltage drop due to wiring resistance is greatly improved. However, the practical response speed is reduced, resulting in deteriorated quality of a displayed image. If an area of a display screen is enlarged or the number of pixels in the display screen is increased, the parasitic capacity is increased, thus the above problem has become more evident.